Joining of component parts together with a fused glass seal to form a composite article is a well cultivated art. In particular, numerous special sealing glasses have been developed for use in joining glass parts with each other, or with metals, alloys, or ceramics.
In making a fusion-type seal, the material must be heated to a temperature where it becomes soft enough to wet the sealing surface and form an adherent, hermetic bond. For many purposes, it is desirable to maintain the sealing temperature as low as possible. This is particularly true in electrical and electronic articles where thermally sensitive parts or coatings are commonly employed.
Accordingly, considerable attention has been given to lead glasses as low temperature sealing glasses. For example, stable sealing glasses, having softening points in the 430.degree.-500.degree. C. range and coefficients of thermal expansion in the 70-90.times.10.sup.-7 /.degree.C. range, are disclosed in U.S. Pat. No. 2,642,633 (Dalton). Subsequent studies centered on lead-zinc borate type glasses that were subject to thermal devitrification or crystallization. These glasses were intensively investigated in the search for cathode ray tube sealing materials.
For many sealing and coating purposes, glasses are used in powder form, termed glass frits. One such application is forming a seal between the funnel and panel members of a cathode ray tube. Sealing glass frits are commonly mixed with an organic vehicle, such as amyl actate, to form a flowable or extrudable paste. This mixture may then be applied to a sealing surface, e.g., the peripheral rim of a funnel or panel member. It is also known to incorporate mill additions in a glass frit mixture, a primary reason being modification and/or control of the ultimate coefficient of thermal expansion in a seal.
A sealing glass frit for joining cathode ray tube parts, in particular the funnel and panel members, has a number of requirements. The primary requirements are excellent flow of the glass at a maximum temperature of 440.degree.-450.degree. C., and low residual strain in the seal. The latter generally requires a coefficient of thermal expansion (CTE) of about 100.times.10.sup.-7 /.degree.C.
In addition to flow and expansion compatibility, a sealing glass frit should possess a number of other favorable characteristics. These include good wettability of the glass parts being sealed, solubility in a common industrial solvent for salvage purposes, and compatibility with organic vehicles. In particular, a frit should be compatible with amyl acetate, a vehicle currently used commercially with lead glass frits.
Industrial frit-sealing processes involve a secondary (post-seal) thermal treatment for metallizing, vacuum exhausting or annealing. This second thermal step is generally performed at a lower temperature than is the sealing operation. However, the temperature involved may be high enough to produce viscous deformation of the frit seal. This brings about distortion and misalignment of the sealed assembly. For these reasons, devitrifying frits are utilized which (ideally) permit an extended interval of glassy flow to occur during the initial stages of the sealing step. Thereafter, the frit crystallizes to become a rigid, deformation-resistant material able to withstand and subsequent thermal processing without distortion.
Frits commonly used for sealing cathode ray tubes are lead-zinc borate sealing glasses, both crystallizing and non-crystallizing. These have been used over a long period of time, and have proven very successful. Nevertheless, there has been a continuing desire for a sealing glass frit having all the favorable characteristics of a lead-zinc borate glass, but with a somewhat lower sealing temperature. Furthermore, as a health and safety measure, strenuous efforts are being made to avoid use of lead compounds as far as possible.
Accordingly, a basic purpose of the present invention is to provide a lead-free sealing glass frit. A further purpose is to provide a lead-free glass having characteristics compatible with forming a fusion seal between the funnel and panel members of a cathode ray tube. Another purpose is to provide a means of altering the effective CTEs of the new lead-free glasses to extend their potential utility to sealing components having lower CTEs. Still another purpose is to provide a mid-temperature sealing glass frit capable of forming a fusion seal at a temperature in the range of 400.degree.-450.degree. C. A still further purpose is to provide a lead-free sealing glass frit that forms a crystallized seal, but that retains good flow properties at 450.degree. C. before crystallizing, and that has a CTE on the order of 100.times.10.sup.-7 /.degree.C.